Sediment archives reveal irreversible shifts in plankton communities after World War II and agricultural pollution

To evaluate the stability and resilience(1) of coastal ecosystem communities to perturbations that occurred during the Anthropocene,(2) pre-industrial biodiversity baselines inferred from paleoarchives are needed.(3,4) The study of ancient DNA (aDNA) from sediments (sedaDNA)(5) has provided valuable information about past dynamics of microbial species(6-8) and communities(9-18) in relation to ecosystem variations. Shifts in planktonic protist communities might significantly affect marine ecosystems through cascading effects,(19-21) and therefore the analysis of this compartment is essential for the assessment of ecosystem variations. Here, sediment cores collected from different sites of the Bay of Brest (northeast Atlantic, France) allowed ca. 1,400 years of retrospective analyses of the effects of human pollution on marine protists. Comparison of sedaDNA extractions and metabarcoding analyses with different barcode regions (V4 and V7 18S rDNA) revealed that protist assemblages in ancient sediments are mainly composed of species known to produce resting stages. Heavymetal pollution traces in sediments were ascribed to the World War II period and coincided with community shifts within dinoflagellates and stramenopiles. After the war and especially from the 1980s to 1990s, protist genera shifts followed chronic contaminations of agricultural origin. Community composition reconstruction over time showed that there was no recovery to a Middle Ages baseline composition. This demonstrates the irreversibility of the observed shifts after the cumulative effect of war and agricultural pollutions. Developing a paleoecological approach, this study highlights how human contaminations irreversibly affect marine microbial compartments, which contributes to the debate on coastal ecosystem preservation and restoration.

Automated summary

This study used sediment cores collected from different sites in the Bay of Brest, northeast Atlantic, France, to retrospectively analyze the effects of human pollution on marine protists over approximately 1,400 years. The research found significant shifts in protist communities after war and agricultural pollution events, with no recovery to a Middle Ages baseline composition, highlighting the irreversible impact of human contaminations on marine microbial compartments.